The field of the invention is that of surge protection systems and the invention relates more particularly to systems having solid state surge protection elements.
Solid state surge protection systems conventionally employ a surge protection device having a semiconducting element between a pair of electrodes. This element is arranged in the circuit to selectively conduct electrical energy between a portion of a telephone circuit and ground, for example, in the event the circuit experiences an electrical surge as a result of lightning or a short circuit or the like in the circuit, thereby to protect electrical components connected in the circuit from damage due to the surge condition. On occasion the circuit will experience an electrical surge which results in destruction of the semiconducting element so that the circuit components would not thereafter be protected against subsequently occurring electrical surge conditions. Accordingly, conventional surge protection systems typically incorporate means to move the electrodes into electrical engagement with each other following destruction of the semiconductor elements, thereby to maintain a short circuit condition between the electrodes to protect the circuit components against subsequently occurring surge conditions until such time as the semiconducting element is replaced. Frequently, difficulty is encountered in establishing and maintaining the desired short circuit condition between the electrodes following destruction of the semiconducting element.
In U.S. Pat. No. 5,224,088, issued Jun. 29, 1993 and assigned to the assignee of the present invention, a surge protection system comprising a circuit and a device is disclosed and claimed which provides first and second levels of surge protection for the circuit and for electrical components in the circuit. The device comprises a semiconducting element having first and second electrodes disposed in electrically conductive relation with respective opposite sides of the element and the element is normally operable to selectively conduct electrical energy between the electrodes to provide a first level of surge protection for the circuit. For example, in one embodiment the electrodes are connected to a portion of the telephone circuit and to ground respectively, and the semiconducting element is non-conductive under normal telephone circuit voltage conditions. The element becomes conductive on occurrence of selected surge voltages in the circuit due to lightning or the like for connecting the circuit to ground to protect the circuit and circuit components from the voltage surge. When the surge condition has passed the semiconducting element returns to its non-conductive condition permitting resumption of normal circuit operation. In that way the surge protection device provides a first level of surge protection for the circuit or system.
The electrodes are adapted to move toward each other to maintain a short circuit condition between the electrodes in the event the device is destroyed as a result of being subjected to a greater electrical surge exceeding the first level of surge protection as described above, thereby to provide a second level of surge protection. Typically for example, the device electrodes are resiliently engaged between spring-loaded electrical contact arms or the like to be biased together by the spring contact arm loading in the event the semiconducting element is destroyed by the noted greater surge conditions. A first one of the electrodes is disposed in electrical engagement with a limited portion of one side of the semiconducting element to selectively concentrate the electrical energy to the limited portion to ensure the establishment and maintenance of the desired short circuit condition between the electrodes in the event of element destruction. The first electrode comprises a disc part and post part of lesser cross-sectional size then the disc part. A distal end of the post part of the first electrode is secured in electrically conductive relation to the selected limited portion of the one side of the element whereby as destruction of the semiconducting element occurs the post part of the first electrode passes through the remains of the element to be electrically engaged in short circuit condition with the second electrode for providing the second level of surge protection. The second electrode has a recess therein and the opposite side of the semiconducting element is received within the recess ensuring alignment in predetermined relation to the second electrode. The disc part of the first electrode is coextensive with the second electrode and the semiconducting element is selected to be operable with either polarity. An environmental seal of an electrically insulative coating or the like is disposed over a portion of said one side of the semiconducting element around the post part of the first electrode. In one embodiment the seal is spaced from the disc part of the first electrode to facilitate movement of the electrodes into short circuit condition in providing the second level of surge protection. In another embodiment the seal is provided with a selected limited thickness and with brittleness to be readily broken by bias of the electrodes toward each other in the event of destruction of the semiconducting element to provide the second level of surge protection. In another embodiment the seal is selected to be destroyed by overheating of the semiconducting element resulting in destruction of the element thereby to facilitate movement of the electrodes short circuit condition.
While the surge protection device in accordance with the teachings of U.S. Pat. No. 5,224,008 is very effective in providing the desired first and second levels of surge protection there is a need to provide an environmental seal which is more easily installed having the desired operational parameters of protecting the semiconducting element and the interfaces of the electrodes from contamination including humidity, salt, dust particles and the like without interfering with the collapsing motion of the electrodes upon destruction of the semiconducting element in providing the second level of surge protection.